Multi-piece solid golf ball

ABSTRACT

The invention provides a distance golf ball endowed with a good flight performance when struck with a driver or an iron by an ordinary amateur golfer, a good durability to repeated impact and a soft feel. The ball is a multi-piece solid golf ball having a core, an intermediate layer and a cover, wherein the core is formed as one or more layer of a rubber composition, the intermediate layer is formed primarily of an ionomer resin, the intermediate layer and the cover are both single layers, each containing an inorganic granular filler, and the ball satisfies the following conditions:surface hardness of ball &gt; surface hardness of sphere consisting of core encased by intermediate layer (intermediate layer-encased sphere),Shore D hardness at surface of ball - Shore D hardness at center of core ≥ 37,specific gravity of cover &gt; specific gravity of intermediate layer.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2022-044029 filed in Japan on Mar. 18, 2022, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a multi-piece solid golf ball having a construction of three of more layers that includes a core, a cover and at least one intermediate layer situated therebetween.

BACKGROUND ART

Up until now, the method generally carried out in order to soften a golf ball --i.e., increase the rebound and improve the feel at impact-- has involved softening the core to achieve the desired feel and encasing the periphery of this core with a hard cover to enhance the rebound. However, although this approach does increase the ball rebound, cracking of the cover upon repeated impact ends up occurring.

Numerous disclosures in which an inorganic filler is added to the cover material have been made in the art relating to golf balls. In some such cases, the main purpose has been to increase the ball’s moment of inertia and enhance its flight performance by increasing the specific gravity of the cover. However, when too much inorganic filler is added, this adversely affects the ball rebound and durability to cracking. Solid golf balls in which the durability of the cover to cracking on repeated impact is greatly improved by adding as a reinforcing material a given amount of an inorganic filler having a certain specific gravity to the cover material have also been described. In addition, JP-A 2000-5341, JP-A 2000-51396, JP-A 2000-51397, JP-A 2000-51398, JP-A 2002-355343, JP-A 2002-355344, JP-A 2000-60997, JP-A 2000-60999, JP-A 2000-70409, JP-A 2000-70411, JP-A 2001-79116, JP-A 2001-340494, JP-A 2003-759, JP-A 2003-760, JP-A 2003-761, JP-A 2003-762, JP-A 2003-126298, JP-A 2004-97802, JP-A 2008-194521, JP-A 2008-194524, JP-A 2011-92708, JP-A 2012-148072 and JP-A 2003-250931 disclose golf balls possessing a good balance of distance, feel at impact and durability to cracking that are obtained by including a granular inorganic filler in the intermediate layer or cover of a multi-piece solid golf ball having a core, an intermediate layer and a cover.

However, these golf balls, when used by ordinary amateur golfers whose head speeds are not as high as those of professional golfers and skilled amateur golfers, are unlikely to exhibit an outstanding flight performance, a good, soft feel at impact and excellent durability to cracking on repeated impact.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a golf ball which has a good flight performance when struck with a driver (W#1) or with an iron by an ordinary amateur golfer, and which moreover has a good durability to repeated impact and a soft feel at impact.

As a result of intensive investigations, I have discovered that, in a multi-piece solid golf ball with a core, an intermediate layer and a cover, certain desirable effects can be achieved by having the core formed as one or more layer of a rubber composition and having the intermediate layer composed primarily of an ionomer resin, formulating the intermediate layer and the cover such that each includes an inorganic granular filler, and setting the surface hardness of the ball to a higher value than the surface hardness of the sphere consisting of the core encased by the intermediate layer (intermediate layer-encased sphere), and moreover by endowing the ball with distance ball properties which place a premium on distance regardless of the spin performance of the ball in the short game and also by setting the specific gravity of the cover to a higher value than that of the intermediate layer. Namely, such a golf ball is able to achieve a good distance on shots taken by an ordinary amateur golfer with a driver or an iron and also has a good durability on repeated impact.

That is, the golf ball of the invention is a distance-type multi-piece solid golf ball which prioritizes distance without regard to the spin performance in the short game. It has a ball construction which includes a hard cover, an intermediate layer composed primarily of ionomer that is softer than the cover, and a rubber core made of one or more layers. In addition, as mentioned above, it has a specific relationship between the specific gravity of the intermediate layer and the specific gravity of the cover. When struck by an ordinary amateur golfer, the ball provides a superior distance performance on full shots with a driver and on full shots with an iron. In addition, an inorganic granular filler is added to the cover and the intermediate layer, which reinforces these layers and increases the durability of the ball to cracking on repeated impact.

Accordingly, in a first aspect, the invention provides a multi-piece solid golf ball having a core, an intermediate layer and a cover, wherein the core is formed as one or more layer of a rubber composition, the intermediate layer is formed primarily of an ionomer resin, the intermediate layer and the cover are both single layers, each containing an inorganic granular filler, and the ball satisfies the following conditions:

-   surface hardness of ball > surface hardness of sphere consisting of     core encased by intermediate layer (intermediate layer-encased     sphere), -   Shore D hardness at surface of ball - Shore D hardness at center of     core ≥ 37, -   specific gravity of cover > specific gravity of intermediate layer.

In a preferred embodiment of the golf ball according to the first aspect of the invention, the cover is composed primarily of an ionomer resin and the intermediate layer is composed primarily of a highly neutralized ionomer resin composition.

In another preferred embodiment of the first aspect of the invention, the Shore D hardness at the surface of the ball is 65 or more.

In yet another preferred embodiment of the first aspect of the invention, the specific gravity of the cover is 1.10 or more and the specific gravity of the intermediate layer is 1.05 or more.

In still another preferred embodiment of the first aspect of the invention, the ball has a core center hardness Cc and a core surface hardness Cs on the Shore C hardness scale such that the value Cs - Cc obtained by subtracting the core center hardness (Cc) from the core surface hardness (Cs) is 12 or more.

In a further preferred embodiment of the first aspect of the invention, the ball has a core surface hardness Cs, a core center hardness Cc and a hardness Cm at a midpoint between the core surface and the core center, all on the Shore C hardness scale, which together satisfy the following condition:

(Cs - Cm)/(Cm - Cc) ≥ 1.2.

In a second aspect, the invention provides a multi-piece solid golf ball having a core, an intermediate layer and a cover, wherein the core is formed as one or more layer of a rubber composition, the intermediate layer is formed primarily of an ionomer resin, and the ball satisfies the following conditions:

-   surface hardness of ball > surface hardness of sphere consisting of     core encased by intermediate layer (intermediate layer-encased     sphere), -   Shore D hardness at surface of ball - Shore D hardness at center of     core ≥ 37, -   specific gravity of cover > specific gravity of intermediate layer,     with the provisos that specific gravity of intermediate layer ≥ 1.05     and specific gravity of cover ≥ 1.10.

In a preferred embodiment of the golf ball according to the second aspect of the invention, the cover is composed primarily of an ionomer resin and the intermediate layer is composed primarily of a highly neutralized ionomer resin composition.

In another preferred embodiment of the second aspect of the invention, the intermediate layer and the cover each contain an inorganic granular filler.

In yet another preferred embodiment of the second aspect of the invention, the Shore D surface hardness of the ball is 65 or more.

In still another preferred embodiment of the second aspect of the invention, the core has a center hardness Cc and a surface hardness Cs on the Shore C hardness scale such that the value Cs - Cc obtained by subtracting the core center hardness (Cc) from the core surface hardness (Cs) is 12 or more.

In a further preferred embodiment of the second aspect of the invention, the ball has a core surface hardness Cs, a core center hardness Cc and a hardness Cm at a midpoint between the core surface and the core center, all on the Shore C hardness scale, which together satisfy the following condition:

(Cs - Cm)/(Cm - Cc) ≥ 1.2.

As used in this Specification, “ordinary amateur golfer” refers to a golfer whose head speed on shots with a driver is in the low-to-medium head speed range of generally from 30 to 43 m/s.

ADVANTAGEOUS EFFECTS OF THE INVENTION

The golf ball of the invention, when struck by an ordinary amateur golfer, enables a superior distance performance to be obtained on full shots with a driver and also enables a superior distance performance to be obtained on full shots with an iron. In addition, the inventive golf ball has a good durability to cracking under repeated impact and moreover has a good, soft feel on impact.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional view of the golf ball according to one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the appended diagram.

Referring to FIG. 1 , the multi-piece solid golf ball of the invention is a golf ball G having a core 1, an intermediate layer 2 encasing the core 1, and a cover 3 encasing the intermediate layer 2. The cover 3 is formed as a single layer. Numerous dimples D are typically formed on the surface of the cover 3. In addition, a coating layer 4 is typically applied to the surface of the cover 3. The cover 3 is positioned as the outermost layer, apart from the coating layer 4, in the layered construction of the golf ball. The core 1 is not limited to a single layer, and may be formed as two or more layers.

The core has a diameter that is preferably 35.9 mm or more, more preferably 36.7 or more, and even more preferably 37.0 mm or more. The upper limit is preferably 39.5 mm or less, more preferably 38.3 mm or less, and even more preferably 37.6 mm or less. When the core diameter is too small, the spin rate on shots with a driver (W#1) may become too high, as a result of which the desired distance may not be attainable by a golfer whose head speed is not fast. On the other hand, when the core diameter is too large, the durability of the ball to repeated impact may worsen.

The deflection of the core when compressed under a load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf), although not particularly limited, is preferably 3.8 mm or more, more preferably 4.0 mm or more, and even more preferably 4.2 mm or more. The upper limit is preferably 5.5 mm or less, more preferably 5.0 mm or less, and even more preferably 4.8 mm or less. When the core deflection is too small, i.e., when the core is too hard, the spin rate of the ball may rise excessively, resulting in a poor distance, or the feel at impact may become too hard. On the other hand, when the core deflection is too large, i.e., when the core is too soft, the ball rebound may become too low, resulting in a poor distance, the feel at impact may become too soft, or the durability of the ball to cracking on repeated impact may worsen.

The core is formed of one or more layer of a vulcanized rubber composition made up primarily of a rubber material, and is preferably formed of a single layer. When the core material is not a rubber material, the rebound decreases, resulting in a poor distance. When the core is formed of a plurality of layers, cracking tends to arise at interfaces between the layers or a loss in initial velocity on full shots may occur, and so care must be taken to avoid large differences in hardness at the interfaces.

The rubber composition of the core is generally composed primarily of a base rubber and includes also other ingredients such as a co-crosslinking agent, a crosslinking initiator, an inert filler and an organosulfur compound.

Polybutadiene is preferably used as the base rubber. A commercial product may be used as the polybutadiene. Examples include BR01, BR51 and BR730, all of which are available from JSR Corporation. The proportion of polybutadiene in the base rubber is preferably 60 wt% or more, and more preferably 80 wt% or more. In addition to this polybutadiene, other rubber ingredients may also be included in the base rubber, insofar as doing so does not adversely affect the advantageous effects of the invention. Examples of such other rubber ingredients include polybutadienes other than the above polybutadiene, and other diene rubbers, including styrene-butadiene rubbers, natural rubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

The co-crosslinking agent is an α,β-unsaturated carboxylic acid and/or a metal salt thereof. Specific examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid and fumaric acid. The use of acrylic acid or methacrylic acid is especially preferred. The metal salt of the unsaturated carboxylic acid is not particularly limited, and is exemplified by the above unsaturated carboxylic acids neutralized with desired metal ions. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, of generally 8 parts by weight or more, preferably 12 parts by weight or more, and more preferably 17 parts by weight or more. The upper limit is generally 50 parts by weight or less, preferably 40 parts by weight or less, and more preferably 30 parts by weight or less. Including too much may make the ball too hard, resulting in an unpleasant feel at impact; including too little may result in a decrease in rebound.

It is preferable to use an organic peroxide as the crosslinking initiator. Specifically, a commercially available organic peroxide may be used. Examples of specific organic peroxides that may be suitably used include Percumyl D, Perhexa C-40 and Perhexa 3M (all available from NOF Corporation), and Luperco 231XL (from AtoChem Co.). These may be used singly or two or more may be used in combination. The organic peroxide content per 100 parts by weight of the base rubber is preferably 0.1 part by weight or more, more preferably 0.3 part by weight or more, and even more preferably 0.5 part by weight or more. The upper limit is preferably 5 parts by weight or less, more preferably 4 parts by weight or less, even more preferably 3 parts by weight or less, and most preferably 2.5 parts by weight or less. A content that is too high or too low may make it impossible to obtain a suitable feel, durability and rebound.

Examples of fillers that may be suitably used include zinc oxide, barium sulfate and calcium carbonate. These may be of one type used alone or two or more may be used in combination. The filler content per 100 parts by weight of the base rubber may be set to preferably 4 parts by weight or more, more preferably 5 parts by weight or more, and even more preferably 7 parts by weight or more. The upper limit of this content per 100 parts by weight of the base rubber may be set to preferably 100 parts by weight or less, more preferably 75 parts by weight or less, and even more preferably 50 parts by weight or less. A content that is too high or too low may make it impossible to obtain a proper weight and a suitable rebound.

Antioxidants that may be used include commercial products such as Nocrac NS-6, Nocrac NS-30, Nocrac 200 and Nocrac MB (all available from Ouchi Shinko Chemical Industry, Co. Ltd.). These may be used singly or two or more may be used in combination.

The antioxidant content per 100 parts by weight of the base rubber, although not particularly limited, is preferably 0.05 part by weight or more, and more preferably 0.1 part by weight or more. The upper limit is preferably 1.0 part by weight or less, more preferably 0.7 part by weight or less, and even more preferably 0.5 part by weight or less. When this content is too high or too low, it may not be possible to obtain a proper core hardness gradient, as a result of which a suitable rebound, durability and spin rate-lowering effect on full shots may not be achievable.

In addition, the rubber composition may include an organosulfur compound in order to impart an excellent resilience. The addition of thiophenols, thionaphthols, halogenated thiophenols or metal salts of these is recommended. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, and any of the following having 2 to 4 sulfur atoms: diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

The organosulfur compound is included in an amount, per 100 parts by weight of the base rubber, of typically 0.03 part by weight or more, preferably 0.05 part by weight or more, and more preferably 0.07 part by weight or more. The upper limit is typically 5 parts by weight or less, preferably 4 parts by weight or less, more preferably 3 parts by weight or less, and most preferably 2 parts by weight or less. When the content is too high, the composition may end up becoming too soft; when the content is too low, an improved resilience is unlikely to be achieved.

The core can be produced by vulcanizing and curing the rubber composition containing the above ingredients. For example, the core can be produced by using a Banbury mixer, roll mill or other mixing apparatus to intensively mix the rubber composition, subsequently compression molding or injection molding the mixture in a core mold, and curing the resulting molded body by suitably heating it under conditions sufficient for the organic peroxide or co-crosslinking agent to act, such as at a temperature of between 100 and 200° C., preferably between 140 and 180° C., for 10 to 40 minutes.

Next, the hardness profile of the core is described. Unless noted otherwise, core hardnesses mentioned below refer to Shore C hardnesses. These Shore C hardnesses are hardness values measured using a Shore C durometer in accordance with ASTM D2240.

The core has a center hardness Cc which is preferably 50 or more, more preferably 55 or more, and even more preferably 58 or more. The upper limit is preferably 66 or less, more preferably 64 or less, and even more preferably 62 or less. When this value is too large, the feel at impact may become hard or the spin rate on full shots may rise, as a result of which the desired distance may not be attainable. On the other hand, when this value is too small, the rebound may decrease, resulting in a poor distance, or the durability to cracking on repeated impact may worsen.

The core has a surface hardness Cs which is preferably 68 or more, more preferably 70 or more, and even more preferably 72 or more. The upper limit is preferably 83 or less, more preferably 80 or less, and even more preferably 78 or less. A core surface hardness outside of this range may lead to undesirable results similar to those described above in connection with the core center hardness Cc.

The core center hardness Cc, when expressed on the Shore D hardness scale (Dc), is preferably 18 or more, more preferably 22 or more, and even more preferably 25 or more. The upper limit is preferably 32 or less, more preferably 30 or less, and even more preferably 28 or less. The core surface hardness Cs, when expressed on the Shore D hardness scale (Ds), is preferably 34 or more, more preferably 36 or more, and even more preferably 38 or more. The upper limit is preferably 48 or less, more preferably 45 or less, and even more preferably 43 or less.

The hardness Cm at the midpoint between the core surface and the core center is preferably 55 or more, more preferably 60 or more, and even more preferably 63 or more. The upper limit is preferably 72 or less, more preferably 70 or less, and even more preferably 68 or less. A core midpoint hardness outside of this range may lead to undesirable results similar to those described above in connection with the core center hardness Cc.

The difference between the core surface hardness Cs and the core center hardness Cc is preferably 12 or more, more preferably 13 or more, and even more preferably 14 or more. The upper limit is preferably 24 or less, more preferably 20 or less, and even more preferably 16 or less. When this value is too small, the spin rate-lowering effect on full shots may be inadequate, resulting in a poor distance. On the other hand, when this value is too large, the initial velocity on shots may decrease, resulting in a poor distance, or the durability of the ball to cracking on repeated impact may worsen.

The ratio of the hardness profile gradient for the surface portion of the core to the hardness profile gradient for the center portion of the core, expressed as (Cs - Cm)/(Cm - Cc), is preferably 1.2 or more, more preferably 1.4 or more, and even more preferably 1.6 or more. The upper limit is preferably 3.0 or less, more preferably 2.4 or less, and even more preferably 1.8 or less. When this ratio is too small, the spin rate-lowering effect on full shots may be inadequate, as a result of which the intended distance may not be achieved. On the other hand, when this ratio is too large, the initial velocity on full shots may decrease, as a result of which the intended distance may not be achieved, or the durability to cracking on repeated impact may worsen.

Next, the intermediate layer is described.

The intermediate layer has a material hardness which is not particularly limited. On the Shore D hardness scale, the material hardness is preferably 40 or more, more preferably 44 or more, and even more preferably 48 or more. The upper limit is preferably 60 or less, more preferably 58 or less, and even more preferably 55 or less. The sphere consisting of the core encased by the intermediate layer (intermediate layer-encased sphere) has a surface hardness on the Shore D hardness scale which is preferably 46 or more, more preferably 50 or more, and even more preferably 54 or more. The upper limit is preferably 66 or less, more preferably 64 or less, and even more preferably 61 or less. When the material hardness and surface hardness of the intermediate layer are lower than the above ranges, the spin rate on full shots may rise excessively, as a result of which a good distance may not be achieved, or the durability to cracking on repeated impact may worsen. On the other hand, when the material hardness and surface hardness of the intermediate layer are higher than the above ranges, the durability of the ball to cracking on repeated impact may worsen or the feel at impact may worsen.

The intermediate layer has a material hardness on the Shore C hardness scale of preferably 63 or more, more preferably 68 or more, and even more preferably 74 or more. The upper limit is preferably 89 or less, more preferably 87 or less, and even more preferably 83 or less.

The intermediate layer has a thickness of preferably from 0.9 to 2.4 mm, more preferably from 1.2 to 2.1 mm, and even more preferably from 1.3 to 1.6 mm. When the intermediate layer thickness falls outside of this range, the spin rate-lowering effect on shots with a driver (W#1) may be inadequate and so a good distance may not be achieved.

An ionomer-based resin composition is used as the intermediate layer material. In particular, for the ball to achieve a superior distance owing to a reduced spin rate on full shots by an ordinary amateur golfer with a driver, it is suitable to use the subsequently described highly neutralized ionomer-based resin composition.

The highly neutralized ionomer-based resin material may be a material in which the essential component is a base resin obtained by blending specific amounts of (a) an olefin-unsaturated carboxylic acid random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid random copolymer with (b) an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer and/or a metal ion neutralization product of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester ternary random copolymer.

Commercial products may be used as components (a) and (b). Illustrative examples of the random copolymer of component (a) include Nucrel™ N1560, Nucrel™ N1214, Nucrel™ N1035 and Nucrel™ AN4221C (all available from Dow-Mitsui Polychemicals Co., Ltd.). Illustrative examples of the random copolymer of component (b) include Nucrel™ AN4311, Nucrel™ AN4318 and Nucrel™ AN4319 (all available from Dow-Mitsui Polychemicals Co., Ltd.).

Examples of metal ion neutralization products of the random copolymer of component (a) include Himilan™ 1554, Himilan™ 1557, Himilan™ 1601, Himilan™ 1605, Himilan™ 1706 and Himilan™ AM7311 (all available from Dow-Mitsui Polychemicals Co., Ltd.), and also Surlyn 7930 (available from The Dow Chemical Company). Examples of metal ion neutralization products of the random copolymer of component (b) include Himilan™ 1855, Himilan™ 1856 and Himilan™ AM7316 (all available from Dow-Mitsui Polychemicals Co., Ltd.), and also Surlyn™ 6320, Surlyn™ 8320, Surlyn™ 9320 and Surlyn™ 8120 (all products of The Dow Chemical Company). Sodium neutralized ionomer resins that are suitable as metal ion neutralization products of the above random copolymers include Himilan™ 1605, Himilan™ 1601 and Himilan™ 1555.

When preparing the base resin, the weight ratio in which component (a) and component (b) are blended can generally be set to from 100:0 to 0:100. Also, the ratio of component (a) to the total amount of components (a) and (b) can be set to preferably 50 wt% or more, more preferably 75 wt% or more, and most preferably 100 wt%.

A non-ionomeric thermoplastic elastomer (e) may be included in the above base resin in order to further enhance the feel of the ball at impact and the rebound. This component (e) is exemplified by olefin elastomers, styrene elastomers, polyester elastomers, urethane elastomers and polyamide elastomers. In this invention, to further increase the rebound, it is desirable to use a polyester elastomer or an olefin elastomer. In particular, an olefin elastomer composed of a thermoplastic block copolymer that contains crystalline polyethylene blocks as the hard segments can be preferably used.

A commercial product may be used as component (e). Specific examples include Dynaron (JSR Corporation) and the polyester elastomer Hytrel® (from DuPont Toray Co., Ltd.).

The content of component (e) may be set to 0 part by weight or more. There is no particular upper limit to the content, although the content per 100 parts by weight of the base resin may be set to preferably 100 parts by weight or less, more preferably 60 parts by weight or less, even more preferably 50 parts by weight or less, and most preferably 40 parts by weight or less. When the content of component (e) is too high, the compatibility of the mixture may decrease, which may lead to a marked decrease in the golf ball durability.

A fatty acid or fatty acid derivative having a molecular weight of at least 228 and not more than 1,500 may be added as component (c) to the base resin. Compared with the base resin, this component (c) has a very low molecular weight. This component suitably adjusts the melt viscosity of the mixture, thereby helping in particular to improve the flow properties. Also, component (c) includes a relatively high content of acid groups (or derivatives thereof), and is able to suppress an excessive loss of resilience.

The amount of component (c) included per 100 parts by weight of the resin component obtained by suitably blending components (a), (b) and (e) may be set to 5 parts by weight or more, preferably 20 parts by weight or more, more preferably 50 parts by weight or more, and even more preferably 70 parts by weight or more. The upper limit in the amount of component (c) may be set to 150 parts by weight or less, preferably 120 parts by weight or less, more preferably 110 parts by weight or less, and even more preferably 100 parts by weight or less. When the amount of component (c) included is too low, the melt viscosity may decrease, lowering the processability; when the amount included is too high, the durability may decrease.

A basic inorganic metal compound capable of neutralizing acid groups in the base resin and component (c) may be added as component (d). By including component (d), acid groups in the base resin and component (c) are neutralized and, owing to synergistic effects that arise from blending these components, the thermal stability of the resin composition rises. At the same time, a good moldability is imparted, making it possible to enhance the resilience of the molded product.

The amount of component (d) included per 100 parts by weight of the resin component may be set to 0.1 part by weight or more, preferably 0.5 part by weight or more, more preferably 1 part by weight or more, and even more preferably 2 parts by weight or more. The upper limit may be set to 17 parts by weight or less, preferably 15 parts by weight or less, more preferably 13 parts by weight or less, and even more preferably 10 parts by weight or less. Including too little component (d) may fail to improve thermal stability and resilience, whereas including too much may instead lower the heat resistance of the golf ball material owing to the presence of excess basic inorganic metal compound.

By including, as mentioned above, specific amounts of components (c) and (d) with respect to the resin component composed of the base resin obtained by blending specific amounts of components (a) and (b) in admixture with optional component (e), a material of excellent thermal stability, flow properties and moldability can be obtained, and the resilience of the resulting molded product can be dramatically improved.

It is recommended that the material obtained by blending specific amounts of the resin component and components (c) and (d) have a high degree of neutralization (i.e., that it be highly neutralized). Specifically, it is recommended that 50 mol% or more, preferably 60 mol% or more, more preferably 70 mol% or more, and even more preferably 80 mol% or more, of the acid groups in the material be neutralized. High neutralization of acid groups in the material makes it possible to more reliably suppress the exchange reactions that cause trouble when only a base resin and a fatty acid (or fatty acid derivative) are used as in the above-cited prior art, thus preventing the generation of fatty acids. As a result, the thermal stability is greatly improved and the moldability is good, enabling molded products to be obtained which have an excellent resilience compared with conventional ionomer resins.

Here, “degree of neutralization” refers to the degree of neutralization of acid groups present within the mixture of the base resin and the fatty acid (or fatty acid derivative) serving as component (c), and differs from the degree of neutralization of the ionomer resin itself in cases where an ionomer resin is used as the metal ion neutralization product of a random copolymer in the base resin. On comparing such a mixture having a certain degree of neutralization with an ionomer resin alone having the same degree of neutralization, the mixture, by including component (d), contains a very large number of metal ions and thus has a higher density of ionic crosslinks which contribute to improved resilience, making it possible to confer the molded product with an excellent resilience.

Optional additives may be suitably included in the intermediate layer material in accordance with the intended application. For example, various additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may be added. When such additives are included, the amount thereof per 100 parts by weight of components (a) to (e) combined is preferably 0.1 part by weight or more, and more preferably 0.5 part by weight or more; the upper limit is preferably 10 parts by weight or less, and more preferably 4 parts by weight or less.

The intermediate layer material may include an inorganic granular filler. This inorganic granular filler is not particularly limited. For example, zinc oxide, barium sulfate, titanium dioxide or the like may be suitably used. From the standpoint of excellent durability to cracking on repeated impact, barium sulfate is preferred. The use of precipitated barium sulfate is especially preferred.

The average particle size of the inorganic granular filler, although not particularly limited, may be set to preferably from 0.01 to 100 µm, and more preferably from 0.1 to 10 µm. When the average particle size of the inorganic granular filler is too small or too large, the dispersibility during preparation of the material may worsen. The average particle size refers here to the value obtained by dispersing the filler together with a suitable dispersant in an aqueous solution, and measuring the particle size with a particle size analyzer.

The content of the inorganic granular filler, although not particularly limited, may be set to preferably 5 parts by weight or more, more preferably 10 parts by weight or more, and even more preferably 15 parts by weight or more, per 100 parts by weight of the base resin of the intermediate layer material. Although there is no particular upper limit, the content is typically 45 parts by weight or less, preferably 35 parts by weight or less, and more preferably 25 parts by weight or less. When the content of inorganic filler is too small, the durability to cracking on repeated impact may worsen. On the other hand, when the content is too large, the ball rebound may become too low and a good distance may not be achieved.

The intermediate layer has a specific gravity which is preferably 1.05 or more, more preferably 1.07 or more, and even more preferably 1.08 or more. The upper limit is preferably 1.20 or less, more preferably 1.15 or less, and even more preferably 1.10 or less. When the specific gravity of the intermediate layer is too small, the durability to cracking on repeated impact may worsen. On the other hand, when the specific gravity of the intermediate layer is too large, the ball rebound may become too low and a good distance may not be achieved.

Next, the cover is described.

The cover has a material hardness which is not particularly limited. On the Shore D hardness scale, the cover material hardness is preferably 59 or more, more preferably 60 or more, and even more preferably 61 or more. The upper limit is preferably 70 or less, more preferably 68 or less, and even more preferably 65 or less. The sphere consisting of the intermediate layer-encased sphere encased by the cover has a surface hardness (ball surface hardness) on the Shore D hardness scale which is preferably 65 or more, more preferably 66 or more, and even more preferably 67 or more. The upper limit is preferably 76 or less, more preferably 74 or less, and even more preferably 71 or less. When the cover material hardness and the ball surface hardness are lower than the above ranges, the spin rate rises on shots with a driver and the ball initial velocity decreases, as a result of which a good distance may not be achieved. On the other hand, when the material hardness of the cover and the ball surface hardness are higher than the above ranges, the durability to cracking on repeated impact may worsen.

The material hardness of the cover on the Shore C hardness scale is preferably 88 or more, more preferably 89 or more, and even more preferably 91 or more. The upper limit is preferably 100 or less, more preferably 98 or less, and even more preferably 96 or less.

The cover has a thickness of preferably 0.8 mm or more, more preferably 1.1 mm or more, and even more preferably 1.3 mm or more. The upper limit in the cover thickness is preferably 1.7 mm or less, more preferably 1.5 mm or less, and even more preferably 1.4 mm or less. When the cover is too thick, the spin rate on shots with a driver may become too high, resulting in a poor distance, or the feel at impact in the short game and on shots with a putter may become too hard. On the other hand, when the cover is too thin, the durability of the ball to cracking on repeated impact may worsen.

A resin material of the same type as or of a different type from that in the above-described intermediate layer material may be used as the chief material of the cover; the use of an ionomer resin is preferred. By using an ionomer resin in the cover material, it is possible to achieve both a higher ball rebound and a lower spin rate on full shots. By using urethane or a rubber material as the cover material, the cover hardness becomes lower and the spin rate rises on full shots, making it difficult to achieve a good distance.

An inorganic granular filler may be included in the cover material. The type and particle size of this inorganic granular filler are the same as for the inorganic granular filler included in the intermediate layer material described above.

The content of the inorganic granular filler per 100 parts by weight of the base resin of the cover material, although not particularly limited, may be set to preferably 10 parts by weight or more, more preferably 15 parts by weight or more, and even more preferably 20 parts by weight or more. Although there is no particular upper limit, this content is preferably 50 parts by weight or less, more preferably 40 parts by weight or less, and even more preferably 30 parts by weight or less. When the content of the inorganic granular filler is too low, the durability of the ball to cracking under repeated impact may worsen. On the other hand, when the content of the inorganic granular filler is too large, the ball rebound may become too low, as a result of which a good distance may not be achieved.

The specific gravity of the cover is preferably 1.10 or more, more preferably 1.12 or more, and even more preferably 1.14 or more. The upper limit is preferably 1.25 or less, more preferably 1.20 or less, and even more preferably 1.16 or less. When the cover specific gravity is too small, the durability of the ball to cracking on repeated impact may worsen. On the other hand, when the cover specific gravity is too large, the ball rebound may decrease excessively, as a result of which a good distance may not be achieved.

The ball has a deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) which, although not particularly limited, is preferably 3.0 mm or more, more preferably 3.2 mm or more, and even more preferably 3.4 mm or more. The upper limit is preferably 4.5 mm or less, more preferably 4.2 mm or less, and even more preferably 4.0 mm or less. When the ball deflection is too small, i.e., when the overall ball is too hard, the spin rate of the ball may rise excessively, resulting in a poor distance, or the feel at impact may become too hard. On the other hand, when the ball deflection is too large, i.e., when the overall ball is too soft, the ball rebound may become too low, resulting in a poor distance, the feel at impact may become too soft, or the durability to cracking on repeated impact may worsen.

In this invention, the relationship between the surface hardness of the ball and the surface hardness of the sphere consisting of the core encased by the intermediate layer (intermediate layer-encased sphere) is such that the surface hardness of the ball is higher than the surface hardness of the intermediate layer-encased sphere. The value obtained by subtracting the surface hardness of the intermediate layer-encased sphere from the surface hardness of the ball, expressed on the Shore D hardness scale, is greater than 0, preferably 5 or more, and more preferably 10 or more. The upper limit is preferably 25 or less, more preferably 20 or less, and even more preferably 15 or less. When this value is too small, the spin rate on full shots with a driver and with an iron rises, as a result of which the intended distance may not be achieved. On the other hand, when this value is too large, the durability of the ball to cracking on repeated impact may worsen.

The value obtained by subtracting the surface hardness of the core from the surface hardness of the ball, expressed on the Shore D hardness scale, is preferably 16 or more, more preferably 20 or more, and even more preferably 24 or more. The upper limit is preferably 36 or less, more preferably 32 or less, and even more preferably 28 or less. When this value is too small, the spin rate on full shots with a driver and with an iron rises, as a result of which the intended distance may not be obtained. On the other hand, when this value is too large, the durability of the ball to cracking on repeated impact may worsen.

The value obtained by subtracting the core center hardness from the ball surface hardness, expressed on the Shore D hardness scale, is preferably 37 or more, more preferably 38 or more, and even more preferably 39 or more. The upper limit is preferably 46 or less, more preferably 44 or less, and even more preferably 42 or less. When the value obtained by subtracting the core center hardness from the ball surface hardness, as expressed on the Shore D hardness scale, is 37 or more, this signifies a ball construction in which the cover is relatively hard and the core is relatively soft compared with conventional distance golf balls. When this value is too small, the spin rate of the ball on full shots with a driver and with an iron rises, as a result of which the intended distance may not be achieved. On the other hand, when this value is too large, the durability of the ball to cracking on repeated impact may worsen.

Specific Gravity Relationship Between Cover and Intermediate Layer

The present invention relates to a multi-piece golf ball that prioritizes distance, in which ball the specific gravity of the cover has been set higher than the specific gravity of the intermediate layer. In cases where an inorganic granular filler is not added to the intermediate layer and the cover, it is the cover material, which is harder than the intermediate layer material, that tends to become the starting point of cracking when the ball is repeatedly struck. It is thus desirable to add a somewhat higher amount of inorganic granular filler to the cover material that tends to crack; doing so has a reinforcing effect on the cover. At the same time, it is desirable to add a somewhat smaller amount of inorganic granular filler to the intermediate layer material than to the cover material. When too much inorganic granular filler is included in both the intermediate layer and the cover, the ball rebound may decrease or the spin rate on full shots may rise, as a result of which a good distance may not be achieved. When the content of inorganic granular filler is too small, the durability of the ball to cracking on repeated impact may decrease.

The difference between the cover specific gravity and the intermediate layer specific gravity, i.e., the value obtained by subtracting the specific gravity of the intermediate layer from the specific gravity of the cover, must be larger than 0, and is preferably 0.02 or more, and more preferably 0.04 or more. The upper limit is preferably 0.20 or less, more preferably 0.15 or less, and even more preferably 0.10 or less. When this value falls outside of the above range, the durability of the ball to cracking on repeated impact may worsen.

Numerous dimples may be formed on the outside surface of the cover. The number of dimples arranged on the cover surface, although not particularly limited, is preferably at least 250, more preferably at least 300, and even more preferably at least 320. The upper limit is preferably not more than 440, more preferably not more than 400, and even more preferably not more than 360. When the number of dimples is higher than this range, the ball trajectory may become lower and the distance traveled by the ball may decrease. On the other hand, when the number of dimples is lower that this range, the ball trajectory may become higher and a good distance may not be achieved. The arrangement of these dimples may have symmetry that follows a tetrahedral, octahedral, dodecahedral or other polyhedral/polygonal shape, or may have rotational symmetry about an axis connecting the poles of the ball.

It is recommended that preferably two or more dimple types, and more preferably three or more dimple types, of mutually differing diameter and/or depth be formed. With regard to the planar shapes of the dimples, a single dimple shape or a combination of two or more dimple shapes, such as circular shapes, various polygonal shapes, dewdrop shapes and oval shapes, may be suitably used. For example, when circular dimples are used, the dimple diameter may be set to at least about 2.5 mm and up to about 6.5 mm, and the dimple depth may be set to at least 0.07 mm and up to 0.30 mm. The cross-sectional shapes of the dimples may be defined as one or a combination of two or more types, including arcuate shapes, conical shapes, flat-bottomed shapes and curves expressed by various functions, and may have, other than near the dimple edges, a plurality of inflection points.

In order for the aerodynamic properties to be fully manifested, it is desirable for the dimple coverage ratio, i.e., the dimple surface coverage SR, which is the collective surface area of the imaginary spherical surfaces circumscribed by the edges of the individual dimples, as a percentage of the spherical surface area of the golf ball, to be set to at least 70% and not more than 90%. Also, to optimize the ball trajectory, it is desirable for the value V₀, defined as the spatial volume of the individual dimples below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, to be set to at least 0.35 and not more than 0.80. Moreover, it is preferable for the ratio VR of the sum of the volumes of the individual dimples, each formed below the flat plane circumscribed by the edge of the dimple, with respect to the volume of the ball sphere were the ball to have no dimples on its surface, to be set to at least 0.6% and not more than 1.0%. Outside the above ranges in these respective values, the resulting trajectory may not enable a good distance to be achieved and so the ball may fail to travel a fully satisfactory distance. Also, to satisfy the rule for symmetry of the ball’s carry, dimple volumes near the poles may be made smaller and dimple volumes near the equator may be made larger than the volumes of dimples away from the poles and the equator.

A coating layer may be formed on the cover surface. This coating layer can be applied using any of various types of coatings. Given the need for the coating to endure the harsh conditions of golf ball use, it is preferable to use a coating composition made up primarily of a urethane coating composed of a polyol and a polyisocyanate.

The thickness of the coating layer composed of the above coating composition, although not particularly limited, is generally from 5 to 40 µm, and preferably from 10 to 20 µm. As used herein, “coating layer thickness” refers to the thickness of the applied coat as determined by measuring the thickness at a total of three places --the center of a dimple and two places located between the center of the dimple and the dimple edgeand averaging the measured values.

When the above coating composition is used, the formation of a coating layer on the surface of golf balls manufactured by a known method can be carried out via the steps of preparing the coating composition at the time of application, applying the composition onto the golf ball surface by a conventional coating operation, and drying the applied composition. The coating method is not particularly limited. For example, spray painting, electrostatic painting or dipping may be suitably used.

The multi-piece solid golf ball of the invention can be made to conform to the Rules of Golf for play. The inventive ball may be formed to a diameter which is such that the ball does not pass through a ring having an inner diameter of 42.672 mm, and to a weight which is preferably between 45.0 g and 45.93 g.

EXAMPLES

The following Examples and Comparative Examples are provided to illustrate the invention, and are not intended to limit the scope thereof.

Examples 1 to 4, Comparative Examples 1 to 7 Formation of Core

Solid cores were produced by preparing the rubber compositions for Example 1 and Comparative Examples 1 to 3 shown in Table 1, and then vulcanizing the compositions under the vulcanization conditions for each Example that are shown in Table 1.

In Examples 2 to 4 and Comparative Examples 4 to 7, a core is produced in the same way as described above using the formulation shown in Table 1.

TABLE 1 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Core Formulation (pbw) Polybutadiene 100 100 100 100 100 100 100 100 100 100 100 Zinc acrylate 20.3 21.0 21.9 19.9 20.4 19.2 20.4 19.2 19.2 26.0 21.9 Organic peroxide (1) 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Organic peroxide (2) 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 Zinc stearate 5 5 5 5 5 5 5 5 5 5 5 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc oxide 5 5 5 5 5 5 5 5 5 5 5 Barium sulfate 13.9 13.6 13.1 14.1 22.0 22.5 19.5 22.5 27.4 11.2 13.1 Zinc salt of pentachlorothiophenol 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 Vulcanization Temperature (°C) 155 155 155 155 155 155 155 155 155 155 155 Time (minutes) 14.5 14.5 14.5 14.5 14.5 14.5 14.5 14.5 14.5 14.5 14.5

Details on the ingredients mentioned in Table 1 are given below.

Polybutadiene: Available under the trade name “BR 01” from JSR Corporation Zinc acrylate: Available under the trade name “ZN-DA85S” from Nippon Shokubai Co., Ltd. Organic Peroxide (1): Dicumyl peroxide, available under the trade name “Percumyl D” from NOF Corporation Organic Peroxide (2): Mixture of 1,1-di(t-butylperoxy)cyclohexane and silica, available under the trade name “Perhexa C-40” from NOF Corporation Zinc stearate: Available under the trade name “Zinc Stearate G” from NOF Corporation Antioxidant: 2,2′-Methylenebis(4-methyl-6-butylphenol), available under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical Industry Co., Ltd. Zinc oxide: Available under the trade name “Sanshu Sanka Aen” from Sakai Chemical Co., Ltd. Barium sulfate: Available under the trade name “Barico #300W” from Hakusui Tech Zinc salt of pentachlorothiophenol: Available from Wako Pure Chemical Industries Co., Ltd.

Formation of Intermediate Layer and Cover (Outermost Layer)

Next, in Example 1 and Comparative Examples 1 to 3, the intermediate layer was formed by using an injection mold to injection-mold resin material No. 1 or No. 2 for the intermediate layer shown in Table 2 over the core surface. The cover was then formed by using a different injection mold to injection-mold resin material No. 3 or No. 4 for the cover (outermost layer) shown in Table 2 over the intermediate layer-encased sphere.

In Examples 2 to 4 and Comparative Examples 4 to 7, the intermediate layer is formed by using an injection mold to injection-mold resin material No. 1 or No. 2 for the intermediate layer shown in Table 2 over the core surface. The cover is then formed by using a different injection mold to injection-mold resin material No. 3, No. 5 or No. 6 for the cover (outermost layer) shown in Table 2 over the intermediate layer-encased sphere.

TABLE 2 Resin composition (pbw) No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 Nucrel™ AN4319 100 100 Himilan™ 1557 50 50 12.5 Himilan™ 1555 50 50 Himilan™ 1605 37.5 Himilan™ 1706 18.75 Himilan™ 1601 12.5 AM 7329 18.75 AM 7327 50 Surlyn™ 8320 50 Magnesium stearate 100 100 1 1 1.7 1 Magnesium oxide 2.8 2.8 Barium sulfate 20 30 25 30 Titanium oxide 2 2 3 2 Ultramarine blue 0.05 0.05 0.1 0.05

Details on the compounding ingredients in the above tables are given below.

Nucrel™ AN4319: A ternary copolymer available from Dow-Mitsui Polychemicals Co., Ltd. Himilan™ 1555, Himilan™ 1557, Himilan™ 1601, Himilan™ 1605 and Himilan™ 1706: Ionomer resins available from Dow-Mitsui Polychemicals Co., Ltd. AM7329 and AM7327: Ionomer resins available from Dow-Mitsui Polychemicals Co., Ltd. Surlyn™ 8320: An ionomer resin available from The Dow Chemical Company Magnesium stearate: Available as “Zinc Stearate G” from NOF Corporation Magnesium oxide: Available as Kyowamag™ MF150 from Kyowa Chemical Industry Co., Ltd. Barium sulfate: Available as “Precipitated Barium Sulfate 300” from Sakai Chemical Industry Co., Ltd. Titanium oxide: Available as “Tipaque R550” from Ishihara Sangyo Kaisha, Ltd.

Each of the golf balls obtained is evaluated by the following methods for various properties, including the respective hardnesses at the surface, center, etc. of the core, the diameters of the core and each layer-encased sphere, the thicknesses and material hardnesses of the respective layers, and the surface hardness of each layer-encased sphere. The results are shown in Table 3.

Diameters of Core and Intermediate Layer-Encased Sphere

The spheres to be measured are held isothermally for at least 3 hours in a thermostatic chamber adjusted to 23.9±1° C., following which they are measured in a 23.9±2° C. room. The diameters at five random places on the surface of each sphere are measured and, using the average of these measurements as the measured value for a single sphere, the average diameter for ten spheres is determined.

Ball Diameter

The balls to be measured are held isothermally for at least 3 hours in a thermostatic chamber adjusted to 23.9±1° C., following which they are measured in a 23.9±2° C. room. The diameters at 15 random dimple-free areas are measured and, using the average of these measurements as the measured value for a single ball, the average diameter for ten balls is determined.

Core and Ball Deflections

The cores or balls to be measured are held isothermally for at least 3 hours in a thermostatic chamber adjusted to 23.9±1° C., following which they are measured in a 23.9±2° C. room. The core or ball is placed on a hard plate and the amount of deflection when compressed under a final load of 1,275 N (130 kgf) from an initial load of 98 N (10 kgf) is measured. The rate at which pressure is applied by the head which compresses the core or ball is set to 10 mm/s.

Core Hardness Profile

The core has a spherical surface. The indenter of a durometer is set substantially perpendicular to this spherical surface, and the surface hardness Cs of the core is measured on the Shore C hardness scale in accordance with ASTM D2240. The core center hardness Cc and the hardness Cm at the midpoint between the core center and the core surface are measured by perpendicularly pressing the indenter of the durometer against the center and the midpoint on a flat cross-section obtained by cutting the core into hemispheres. The hardnesses at the center and at the midpoint are shown as Shore C hardness values. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) equipped with a Shore C durometer can be used for measuring the hardness. The maximum value is read off as the hardness value. All measurements are carried out in a 23±2° C. environment. The numerical values in Table 3 are Shore C hardness values.

Material Hardnesses of Intermediate Layer and Cover

The resin material for each layer is molded into a sheet having a thickness of 2 mm and left to stand for at least two weeks. The Shore C and Shore D hardnesses are then measured in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) can be used to measure the hardness. Attachments for the Shore C hardness and the Shore D hardness are mounted on the tester and the respective hardnesses are measured. The maximum value is read off as the hardness value. All measurements are carried out in a 23±2° C. environment.

Surface Hardnesses of Intermediate Layer-Encased Sphere and Ball

These surface hardnesses are measured by pressing the indenter of the durometer perpendicularly against the surface of the sphere to be measured. The surface hardness of the ball (cover) is the measured value obtained at land areas on the ball surface where dimples are not formed. The Shore D hardness is measured in accordance with ASTM D2240. The P2 Automatic Rubber Hardness Tester (Kobunshi Keiki Co., Ltd.) can be used to measure the surface hardnesses. The maximum value is read off as the hardness value. All measurements are carried out in a 23±2° C. environment.

TABLE 3 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Ball construction (piece) 3P 3P 3P 3P 3P 3P 3P 3P 3P 3P 3P Core Diameter (mm) 37.36 37.36 37.36 37.36 37.33 37.36 37.33 37.36 37.30 37.36 37.36 Weight (g) 30.77 30.77 30.77 30.77 31.96 32.02 31.56 32.02 32.60 30.77 30.77 Specific gravity 1.13 1.13 1.13 1.13 1.17 1.17 1.16 1.17 1.20 1.13 1.13 Deflection (mm) 4.7 4.5 4.2 4.8 4.6 4.8 4.5 4.8 4.8 3.5 4.2 Core hardness profile Core surface hardness: Cs (Shore C) 75 77 78 75 76 74 76 74 74 83 78 Core midpoint hardness: Cm (Shore C) 65 67 68 65 66 65 66 65 65 73 68 Core center hardness: Cc (Shore C) 60 61 62 59 60 59 60 59 59 66 62 Cs - Cc 15 16 16 16 16 15 16 15 15 17 16 Cs - Cm 10 10 10 10 10 9 10 9 9 10 10 Cm - Cc 5 6 6 6 6 6 6 6 6 7 6 (Cs - Cm)/(Cm - Cc) 2.0 1.7 1.7 1.7 1.7 1.5 1.7 1.5 1.5 1.4 1.7 Core surface hardness: Ds (Shore D) 40 42 43 40 41 39 41 39 39 48 43 Core center hardness: Dc (Shore C) 27 28 28 26 27 26 27 26 26 32 28 Intermediate layer Material No.1 No.1 No.1 No. 1 No.2 No.2 No.2 No.1 No.2 No. 1 No. 1 Thickness (mm) 1.34 1.34 1.34 1.34 1.34 1.33 1.34 1.32 1.35 1.34 1.34 Specific gravity 1.09 1.09 1.09 1.09 0.96 0.96 0.96 1.09 0.96 1.09 1.09 Sheet (material) hardness (Shore D) 49 49 49 49 48 48 48 49 48 49 49 Sheet (material) hardness (Shore C) 80 80 80 80 79 79 79 80 79 80 80 Intermediate Later- encased sphere Diameter (mm) 40.04 40.04 40.04 40.04 40.01 40.01 40.01 40.00 40.00 40.04 40.04 Weight (g) 37.61 37.61 37.61 37.61 37.96 37.98 37.65 38.78 38.80 37.61 37.61 Surface hardness (Shore D) 55 55 55 55 54 54 54 55 54 55 55 Cover Material No.3 No.3 No.3 No. 3 No.4 No.4 No.3 No.5 No.5 No. 3 No.6 Thickness (mm) 1.34 1.34 1.34 1.34 1.35 1.35 1.35 1.35 1.35 1.34 1.34 Specific gravity 1.14 1.14 1.14 1.14 1.12 1.12 1.14 0.97 0.97 1.14 1.14 Sheet (material) hardness (Shore D) 62 62 62 62 61 61 62 62 62 62 42 Sheet (material) hardness (Shore C) 93 93 93 93 94 94 93 94 94 93 72 Ball Surface hardness 68 68 68 68 67 67 68 68 68 68 48 Diameter 42.72 42.72 42.72 42.72 42.71 42.71 42.71 42.70 42.70 42.72 42.72 Weight 45.25 45.25 45.25 45.25 45.32 45.37 45.18 45.38 45.40 45.25 45.25 Deflection (mm) 3.77 3.64 3.48 3.83 3.75 3.90 3.74 3.85 3.90 2.91 3.83 Ball surface hardness - Intermediate layer surface hardness (Shore D) 13 13 13 13 13 13 14 13 14 13 -7 Ball surface hardness - Core surface hardness (Shore D) 28 26 25 28 26 28 27 29 29 20 5 Ball surface hardness - Core center hardness (Shore D) 41 40 40 42 40 41 41 42 42 36 20 Cover material specific gravity -Intermediate layer material specific gravity 0.05 0.05 0.05 0.05 0.16 0.16 0.18 -0.12 0.01 0.05 0.05

The flight of each golf ball on shots with a driver (W#1) and a number six iron (I#6) and the durability on repeated impact are evaluated by the following methods. The results are shown in Table 4.

Evaluation of Flight (W#1)

A driver (W#1) is mounted on a golf swing robot and the spin rate and total distance of the ball when struck at head speeds (HS) of 40 m/s and 35 m/s are measured. The club used is the JGR (2016 model; loft angle, 9.5°), manufactured by Bridgestone Sports Co., Ltd. The evaluation criteria are as follows.

-   Evaluation Criteria at Head Speed of 40 m/s     -   Good: Total distance is 203.0 m or more     -   NG: Total distance is less than 203.0 m -   Evaluation Criteria at Head Speed of 35 m/s     -   Good: Total distance is 168.0 m or more     -   NG: Total distance is less than 168.0 m

Evaluation of Flight (I#6)

A number six iron (I#6) is mounted on a golf swing robot and the spin rate and total distance of the ball when struck at a head speed of 35 m/s are measured. The club used is the JGR Forged (2016 model), manufactured by Bridgestone Sports Co., Ltd.

Evaluation Criteria

-   Good: Total distance is 145.8 m or more -   NG: Total distance is less than 145.8 m

Durability to Cracking on Repeated Impact

The durability of the golf ball is evaluated using an ADC Ball COR Durability Tester manufactured by Automated Design Corporation (U.S.). This tester fires a golf ball pneumatically and causes it to consecutively strike two metal plates arranged in parallel. The average value for the number of shots required for the golf ball to crack is treated as the durability. Here, the average value refers to the value obtained by furnishing three balls of the same type for testing, firing each ball, and averaging the number of shots required for the respective balls to crack. The type of tester used is a horizontal COR tester, and the incident velocity against the metal plates is set to 43 m/s.

Evaluation Criteria

-   Good: Average value is 130 times or more -   NG: Average value is less than 130 times

TABLE 4 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Flight performance W#1 HS, 40 m/s Spin rate (rpm) 2,676 2,683 2,692 2,673 2,648 2,650 2,646 2,676 2,680 2,722 2,775 Total distance (m) 204.9 204.2 203.4 205.3 201.0 202.4 201.1 205.1 205.0 200.3 200.4 Rating good good good good NG NG NG good good NG NG W#1 HS, 35 m/s Spin rate (rpm) 2,791 2,823 2,865 2,775 2,778 2,747 2,758 2,762 2,748 3,009 2,875 Total distance (m) 170.0 169.4 168.7 170.3 169.6 173.3 170.2 170.6 171.0 166.0 165.6 Rating good good good good good good good good good NG NG I#6 HS, 35 m/s Spin rate (rpm) 4,675 4,720 4,778 4,652 4,650 4,629 4,699 4,654 4,655 4,979 5,202 Total distance (m) 146.3 146.1 145.9 146.4 146.1 146.6 145.6 146.2 146.1 145.0 139.0 Rating good good good good good good NG good good NG NG Durability to cracking on repeated impact good good good good NG NG NG NG NG good good

As demonstrated by the results in Table 4, the golf balls of Comparative Examples 1 to 7 are inferior in the following respects to the golf balls according to the present invention that are obtained in Examples 1 to 4.

In Comparative Example 1, an inorganic granular filler is not included in the intermediate layer and the specific gravity of the intermediate layer is less than 1.05. As a result, the ball does not have a good flight on shots with a driver (W#1) at a head speed (HS) of 40 m/s and the durability of the ball to repeated impact is inferior.

In Comparative Example 2, an inorganic granular filler is not included in the intermediate layer and the specific gravity of the intermediate layer is less than 1.05. As a result, the ball does not have a good flight on shots with a driver (W#1) at a head speed (HS) of 40 m/s and the durability of the ball to repeated impact is inferior.

In Comparative Example 3, an inorganic granular filler is not included in the intermediate layer and the specific gravity of the intermediate layer is less than 1.05. As a result, the ball does not have a good flight on shots with a driver (W#1) at a head speed (HS) of 40 m/s and the durability of the ball to repeated impact is poor.

In Comparative Example 4, an inorganic granular filler is not included in the cover and the specific gravity of the cover is lower than the specific gravity of the intermediate layer. As a result, the durability of the ball to repeated impact is poor.

In Comparative Example 5, an inorganic granular filler is not included in both the cover and the intermediate layer, in addition to which the intermediate layer has a specific gravity below 1.05 and the cover has a specific gravity below 1.10. As a result, the durability of the ball to repeated impact is poor.

In Comparative Example 6, the Shore D value obtained by subtracting the core center hardness from the ball surface hardness is 36, which is lower than 37. As a result, the distances traveled by the ball on shots with a driver (W#1) and an iron (I#6) are poor.

In Comparative Example 7, the surface hardness of the ball is lower than the surface hardness of the intermediate layer-encased sphere. In addition, the Shore D value obtained by subtracting the core center hardness from the ball surface hardness is 20, which is lower than 37. As a result, the distances traveled by the ball on shots with a driver (W#1) and an iron (I#6) are poor.

Japanese Patent Application No. 2022-044029 is incorporated herein by reference.

Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims. 

1. A multi-piece solid golf ball comprising a core, an intermediate layer and a cover, wherein the core is formed as one or more layer of a rubber composition, the intermediate layer is formed primarily of an ionomer resin, the intermediate layer and the cover are both single layers, each containing an inorganic granular filler, and the ball satisfies the following conditions: surface hardness of ball > surface hardness of sphere consisting of core encased by intermediate layer (intermediate layer-encased sphere), Shore D hardness at surface of ball - Shore D hardness at center of core ≥ 37, specific gravity of cover > specific gravity of intermediate layer.
 2. The golf ball of claim 1, wherein the cover is composed primarily of an ionomer resin and the intermediate layer is composed primarily of a highly neutralized ionomer resin composition.
 3. The golf ball of claim 1, wherein the Shore D hardness at the surface of the ball is 65 or more.
 4. The golf ball of claim 1, wherein the specific gravity of the cover is 1.10 or more and the specific gravity of the intermediate layer is 1.05 or more.
 5. The golf ball of claim 1, wherein the core has a center hardness Cc and a surface hardness Cs on the Shore C hardness scale such that the value Cs - Cc obtained by subtracting the core center hardness Cc from the core surface hardness Cs is 12 or more.
 6. The golf ball of claim 1, wherein the ball has a core surface hardness Cs, a core center hardness Cc and a hardness Cm at a midpoint between the core surface and the core center, all on the Shore C hardness scale, which together satisfy the following condition: (Cs - Cm)/(Cm - Cc) ≥ 1.2. .
 7. A multi-piece solid golf ball comprising a core, an intermediate layer and a cover, wherein the core is formed as one or more layer of a rubber composition, the intermediate layer is formed primarily of an ionomer resin, and the ball satisfies the following conditions: surface hardness of ball > surface hardness of sphere consisting of core encased by intermediate layer (intermediate layer-encased sphere), Shore D hardness at surface of ball - Shore D hardness at center of core ≥ 37, specific gravity of cover > specific gravity of intermediate layer, with the provisos that specific gravity of intermediate layer ≥ 1.05 and specific gravity of cover ≥ 1.10.
 8. The golf ball of claim 7, wherein the cover is composed primarily of an ionomer resin and the intermediate layer is composed primarily of a highly neutralized ionomer resin composition.
 9. The golf ball of claim 7, wherein the intermediate layer and the cover each contain an inorganic granular filler.
 10. The golf ball of claim 7, wherein the Shore D hardness at the surface of the ball is 65 or more.
 11. The golf ball of claim 7, wherein the core has a center hardness Cc and a surface hardness Cs on the Shore C hardness scale such that the value Cs - Cc obtained by subtracting the core center hardness Cc from the core surface hardness Cs is 12 or more.
 12. The golf ball of claim 7, wherein the ball has a core surface hardness Cs, a core center hardness Cc and a hardness Cm at a midpoint between the core surface and the core center, all on the Shore C hardness scale, which together satisfy the following condition: (Cs - Cm)/(Cm - Cc) ≥ 1.2. . 